The present invention is related to techniques for watermarking digital data, and more specifically, to watermarking digital data such as images and audio data for authenticating copyright ownership.
Because of the rapid increase of electronic commerce in the recent years, secured data transaction is becoming more and more important. To prevent electronic data to be appropriated by unauthorized parties, cryptographic methods have been used to transmit digital data between consenting parties to guard against unintended exposure to other parties. Various kinds of data, including military information, financial transaction, personal data, and the like can be transmitted via cryptography to protect the data.
In the arena of protecting the right to original art or literary works, in the past, most violations were by unauthorized parties making physical copies of authorized copies of the original (e.g., copyrighted) works. Copies of physical artistic material, such as copyrighted paintings, photographs, phonographs, and analog audio tapes, are usually perceptibly inferior to the originals. The degradation of fidelity in the copying process, e.g., in photocopying or photography, is a factor in deterring unauthorized copying of such material. Today, many visual, audio, literary, or other proprietary works are stored and transmitted digitally. Such digital material can be copied over and over without significant loss in fidelity. The risk to the owner of an original artistic work, or proprietary work, is that once the digital data are transmitted, if data suspected to be copies of the transmitted data are found, the verification of whether the suspect data are copied from the originally transmitted data, for example, digital data of a piece of art work, is usually impossible.
Recently, digital watermarking has been devised as a security technique to facilitate the identification of the source of digital material for the purpose of, for example, copyright enforcement. The watermark is an identification code that is imbedded in the original digital data and is preferably imperceptible to the human observer of the artistic work. One example of a scheme for watermarking involves inserting an identification string into a digital audio signal to substitute the insignificant bits of randomly selected audio samples with the bits of an identification code. Another example of watermarking relating to watermarking video digital works involves assigning a predetermined value to a predetermined coding parameter that, when modified, requires a plurality of further parameters to be modified in order to correctly decode the video signal. In one watermarking technique each copy of an object is marked with an identifier code. More recently, a watermarking scheme in which a two-dimensional spread spectrum signal is added to an image has been proposed. To verify the watermark in a given image, the original image is subtracted from the given image and the correlation of the difference image to the watermark signal is computed.
Although much advance has been made in watermarking digital data, generally, prior techniques of watermarking suffer from a variety of shortcomings. Often, the original image is necessary to verify the presence of the watermark. Manipulative operations such as cropping to cut out a portion of the work and scaling to obtain a work of larger or smaller size pose a considerable problem on the verification process. Many times the watermarking scheme is invertible, i.e., an attacker (or temperer), based on a first watermarked image but without knowledge of what the first watermark is, would be able to compute a second image and a second watermark such that inserting the second watermark into the second image would result in the first watermarked image. Such invertible watermarking schemes can make the verification of authentic copies of a copyrighted work difficult. Further, it may even lead to ownership disputes on valuable digital data since an unscrupulous person might appropriate another""s watermarked material, subtract from it his own watermark and claim the resulting product to be his own. Often, in prior watermarking techniques, changes in contrast or brightness may fool the verification algorithms, making them less reliable. Furthermore, many of the watermark schemes do not offer a mechanism for creating and managing watermarks. This means that if the same watermark is used to protect several works, compromise in the secrecy of the watermark will compromise the protection for all of the works. What is needed a watermarking technique that can address these deficiencies. The present invention provides a watermarking technique that is resistant to cropping, invertible, resistant to brightness or contrast changes, and will not compromise other related watermarked works if the mechanism of watermarking in one watermarked work is disclosed.
The present invention provides a technique for identifying digital object using a digital watermark. This technique can be easily implemented using computers. The technique includes encrypting data derived from a set of source data on the digital object, deriving from the encrypted data a watermark, and incorporating the watermark into the source data. Preferably, the source data of the digital object are processed through a hash function to obtain a message digest (M) on the digital object and the message digest (M) is encrypted with a signature encryption key to obtain an encrypted message digest (S). Further, the encryption of the message digest (M) is preferably done with a public key-private key encryption system. Because the preferred mode is to process the source data through a one-way hash function to obtain a message digest, for clarity and convenience, the set of data for encryption derived from the set of source data is termed a xe2x80x9cmessagexe2x80x9d or xe2x80x9cmessage digestxe2x80x9d herein although they do not necessarily have to have been through a one-way hash function. The set of encrypted data is called xe2x80x9cencrypted message digestxe2x80x9d herein for the same reason.
The present watermarking technique is versatile and can be advantageously employed to watermark a variety of digital objects, including audio, video, image, multimedia data, and the like. Further, the present technique offers high security because it is not easily foiled by attackers. For example, by a using a hash function in conjunction with encryption in which a private key is kept confidential, the present technique offers the advantage of being not invertible, unlike many prior watermarking techniques. Therefore, it is very difficult for an attacker to backcompute the original watermark by the disclosure of immediate information. To further increase security, in an embodiment of the present invention a public key encryption system is employed. As a result, a suspect object (i.e., an object suspected to be copied from an original, e.g., copyrighted, object) can be checked with the public key of the original object""s owner to determine whether the watermark is present without compromising the other watermarked objects. In the embodiment in which an encryption technique involving a public key is used, ownership of an object can be established to a neutral party, such as a court of law, using only the public key of the object""s owner, without requiring the owner to reveal his private key. Also, the watermark is not removed by lossy compression (in which less perceptually important information is severed to reduce the size of the data file) or cropping (in which a portion of the object is cut). In the embodiment in which the watermarking vector is orthogonal to the vector of the pixels into which the watermark is to be inserted, changes in the brightness or contrast will not fool the verification algorithm.